1. Field of the Invention
The present invention is directed to a spherical linkage, and in particular to a closed loop serial spherical linkage having five interconnected links.
2. Prior Art
Multiple link linkages for moving devices along paths of motion with or without orientation are well known, and may have four, five or more links. Such devices may be used for a number of different functions and are used to move elements through paths of motion, typically around one or more pivots.
Serial type linkages are often used in digitizing applications, to move a device at an extended end of the linkage. Such linkages typically have the various link members connected in an end-to-end relationship, but have an open loop rather than a closed loop. Therefore, such linkages are prone to more imprecision than closed loop linkages due to tolerances building up from the base to the extended end. Although such devices provide great flexibility, each link provides an additional degree of freedom which requires an additional driver, one for controlling each degree of freedom to provide the necessary control at the extended end of the linkage. As more drivers are required for the linkage, the mass of the linkage is increased so that movement is more difficult due to greater inertia. Therefor, the linkage must be designed with a greater load capacity than would otherwise be required.
The imprecision is compounded as the additional drivers which provide control may each add some imprecision associated with stopping the motor at a desired location. Control of such a linkage is difficult as the multiple degrees of freedom must be coordinated for accurate usage. Although such devices may be useful for some applications, they have drawbacks which limit their usage in many applications.
A spherical linkage has been developed for uses such as for rehabilitation of the ankle. Such a device is described in the article entitled xe2x80x9cSpherical Four-Bar Linkage Mechanism for Continuous Passive Movement Rehabilitation Treatment of the Anklexe2x80x9d presented at the 1996 American Society of Mechanical Engineers Design Engineering Technical Conference in Computers and Engineering Technical Conferences and Computers and Engineering Conference from Aug. 18-22, 1996, in Irvine, Calif. The device provides for moving the patient""s ankle in a continuous motion and provides a closed loop linkage. The closed spherical design provides improved support over open serial type linkages. However, the four bar spherical linkage does not provide sufficient degrees of freedom to provide for other paths of motion which may be required with many applications. The linkage can only trace one path of motion for any instrument or device mounted thereon.
Heretofore, devices using spherical linkages of more than four bars have taken the form of parallel linkages, wherein links are connected to one another but not in a serial configuration. An example of such a parallel type linkage is the Agile Eye, developed to support photography and having extremely fast reaction times. The Agile Eye is capable of making adjustments much faster than the human eye. Although the Agile Eye obtains great speeds for changing camera angles, it has a limited range of motion, limiting the camera range. The Agile Eye requires a complicated parallel arrangement with the links nesting within one another. Moreover, the equations defining and controlling parallel linkages are more complicated and difficult to solve than the equations for serial linkages.
It can be seen then that a spherical five-bar closed loop linkage is required which overcomes the shortcomings of the linkages of the prior art. Such a linkage should provide precision and support through a closed loop linkage. In addition, the linkage should provide multiple degrees of freedom and a range of motion which accommodates multiple functions requiring multiple paths of motion. The linkage should also provide movement of devices relative to one another. The linkage should provide for manipulating instruments to access the interior of the sphere defined by the linkage. The present invention addresses these as well as other problems associated with the prior art.
The present invention is directed to a linkage apparatus, and in particular to a spherical five bar closed loop serial linkage. The linkage may be utilized in systems for scanning, surgical procedures, and other applications requiring precise motion.
The spherical linkage defines a sphere having a center through which the axis of rotation through a first joint passes. In addition, the axes of rotation of all the joints between the various links also pass through the same spherical center. The respective lengths, radiuses and angles of the various links may be varied, but each axis of rotation will always intersect with the center of the sphere defined by the linkage. The linkage includes a first link, designated as a ground link having a first end pivotally connected to a first end of a second link. The second end of the second link connects to the first end of the third link at a second joint having an axis of rotation extending through the intersection at the center of the linkage sphere. The second end of the third link connects to the first end of a fourth link through a third joint having an axis of rotation also intersecting at the center of the sphere. The second end of the fourth link connects to a first end of a fifth link to a fourth joint having an axis rotation extending through the center of the sphere. The second end of the fifth link and the second end of the first or ground link connect at a fifth joint having an axis of rotation extending through the center of the sphere.
The lengths of the various links and radius may be varied. Each link is defined by an angular measurement between the two axis of rotations originating at the center of the linkage sphere. For example, a link may be designated as an 80xc2x0 link. As the radius is decreased or increased, the linkage is still operable with that link defining the same angle. The combinations of the various angles will define the rotational ranges for the various angles. For example, with some embodiments, one or more links are able to rotate 360xc2x0, while in some embodiments, no link will be able to rotate 360xc2x0.
The spherical serial closed loop five bar linkage has two degrees of freedom. Therefor, devices utilizing the linkage include two drive motors to position the various links. The drivers may be positioned at adjacent joints or spaced apart, depending on the particular application.
In one application, the linkage is utilized in a scanning device for scanning and digitizing objects. A scanner is positioned apart from the linkage and an object placed within the linkage sphere proximate the center. The spherical linkage provides for moving the scanner and object being scanned relative to one another without the linkage interfering with the scanning beam. For example, dental scanning may be accomplished with the linkage of the present invention. A supported dental mold is passed through the scanner beam at various angles, creating a digital model of the mold.
In another application, the linkage is utilized for supporting instruments for surgery performed on the eye. The linkage of the present inventions allows for making a small puncture at a single point on the surface of the eyeball coinciding with the spherical center of the linkage. In this manner, the instrument extending through the spherical center may be moved through the fluid making up the interior of the eye without creating a tear or larger opening at the surface of the eye.
Other applications may also be utilized that employ non-linear paths of motions along three axes. The linkage of the present invention may be used in other biological and industrial applications suited for linkages that interact with spherical or curved shapes, such as scanning and digitizing objects. A spherical closed loop five bar linkage provides greater strength and support with less flexure than serial open linkages. In addition, a supported instrument may be offset from its supporting link to provide improved access without interference with other links, objects or a scanning beam. The spherical linkage has multiple configurations and link combinations to meet the needs and parameters, of the particular application.
These features of novelty and various other advantages which characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.